The present application relates generally to the field of residential heating, ventilating, and air conditioning (“HVAC”). Residential energy construction codes require houses to be extremely airtight. During the operation of common home appliances such as clothes dryers, kitchen and bath exhaust fans, and fireplaces, air is removed from the interior of a residential building, resulting in a negative pressure gradient between the interior and the exterior of the building. Under those conditions, large amounts of outdoor air containing dusts, pollens, and other pollutants will be drawn into a home any time a door, window, or other air passage is opened to the exterior. There is a need for an inexpensive, simple solution to this problem.